The present invention relates to an electrophotosensitive material which is used in image forming apparatuses utilizing a so-called electrophotographic process, such as electrostatic copying machine, plane paper facsimile, laserprinter and the like, and an image forming method using the same.
Recently, there have widely been used a so-called organic photoconductor (OPC) such as a photoconductor comprising a single-layer type photosensitive layer obtained by dispersing an electric charge generating material capable of generating electric charges (holes and electrons) by light irradiation and an electric charge transferring material capable of transferring the generate electric charges in a single layer made of a binding resin, or a photoconductor comprising a multi-layer type photosensitive layer obtained by laminating an electric charge transferring layer containing an electric charge transferring material and an electric charge generating layer containing an electric charge generating material, as the above electrophotosensitive material.
Such an organic photoconductor has advantages such as easier production than an inorganic photoconductor using a deposited film made of an inorganic semiconductor material, various selective photosensitive materials (e.g. electric charge generating material, electric charge transferring material, binding resin, etc.) and high rate of freedom with functional design.
Examples of the electric charge transferring material a include hole transferring material having excellent transferring capability of holes and electron transferring material having excellent transferring capability of electrons. As the hole transferring material, various organic compounds such as carbazole compound, oxadiazole compound, pyrazoline compound, phenylenediamine compound, benzidine compound and the like are known.
Among them, a m-phenylenediamine compound represented by the general formula (2): ##STR2## wherein R.sup.2A, R.sup.2B, R.sup.2C and R.sup.2D are the same or differentand indicate a hydrogen atom, an alkyl group, an alkoxy group or an aryl group has widely been used, particularly, because of its excellent characteristics as described below.
That is the m-phenylenediamine compound (2) has the following advantages. That is, the transferring capability of holes is excellent because of large drift mobility, which indicates thetransferring capability of holes, and a residual potential is liable to be drawn at low electric field because dependence of the drift mobility on an field intensity is small. Furthermore, the m-phenylenediamine compound is superior in compatibility with a binding resin constituting the electric charge transferring layer and also has light resistance to some extent to ultraviolet light.
However, a photoconductor using the m-phenylenediamine compound (2) had such a problem that unrestorable damage is caused by exposing to a fluorescent lamp for interior illumination or strong light such as sunlight coming into a room through a window in the state where the body of an image forming device is opened for a long time in case of maintenance, or by exposing to strong light described above in the high-temperature state in case of operation even for a short time when the body is opened because paper jam occurs during the operation of the device.
This reason is considered as follows. That is, a photo-deterioration reaction occurs by exposing to strong light described above, specifically a cyclization between a central benzene ring and the other phenyl group, thereby changing the m-phenylenediamine compound (2) into impurities as a trap to transfer of holes.
That is, the density of electrons of the m-phenylenediamine compound (2) is biased against the benzene ring in the molecular center and the compound has such a molecular structure that carbon at the 5-position of above benzene ring is likely to be attacked by an oxidizing substance such as oxygen in case of light excitation because of its configuration. Therefore, it is considered that the above cyclization reaction can occur by drawing electrons from the carbon at the 5-position of above benzine ring.
Further, since a melting point of the m-phenylenediamine compound (2) is generally low, a photosensitive layer obtained by using the compound has a low glass transition temperature and is insufficient in durability and heat resistance. Particularly, when the device stops in the high-temperature state in case of the operation and is allowed to stand for a long time, an impression due to a cleaning blade appears as a striped concave portion on the surface of the photosensitive layer, which can causes image defects.
Therefore, in order to solve these problems, there have been suggested a m-phenylenediamine compound wherein the durability to strong light exposure is improved by substituting a group such as alkyl group on the 5-position of the central benzene ring as shown in the general formula (3) below, and an electrophotosensitive material using the same (Japanese Examined Patent Publication No. 9579/1996). ##STR3## wherein R.sup.3A, R.sup.3B, R.sup.3C and R.sup.3D are the same or different and indicate an alkyl group, an alkoxy group, a halogen atom, an amino group or a N-substituted amino group; A, B, C and D are the same or different and indicate an integer of 0 to 5; and R.sup.3E indicates an alkyl group, an alkoxy group, an amino group, an allyl group or an aryl group.
Since such a m-phenylenediamine compound (3) has characteristics peculiar to a conventional m-phenylenediamine compound (2) described above and has high durability to strong light exposure, it is expected that the performance of the organic electrophotosensitive material can be more improved than the prior art.
Further, especially, a compound, wherein an aryl group such as phenyl group is substituted as the above group R.sup.3E, has a particularly high melting point and, therefore, it is expected that the durability and heat resistance can be improved by rising of the glass transition temperature of the photosensitive layer.
However, in order to satisfy increasing requirements such as realization of more higher speed and much larger energy saving of the image forming device, the sensitivity of an electrophotosensitive material using a conventional hole transferring material including the above m-phenylenediamine compound (3) has already becoming insufficient at present. Therefore, it has been required to develop a novel hole transferring material capable of forming an electrophotosensitive material having a higher sensitivity.